Superheterodyne receiver



MVC,

el frfcfm W. S. BARDEN SUPERHETEEODYNE RECEIVER E7I-0JUZMTOH Jan. 16,1934.

INVENTOR wlw/AM s. EAEDEN ATTORNEY aiented Jan. 16, 1934 Y UNITED STATESPATENT oFFicE 1,944,11r SUPERHETERODYNE RECEIVER William S. Barden,Grasmere, Staten Island, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware My present invention relates tosuperheterodyne receivers, and more particularly to a novel method of,and means for, suppressing undesired responses in radio receivers of thesuperheterodyne type.

It is well known at the present time that a superheterodyne type ofreceiver may be embodied in a comparatively low priced chassis andcabinet. Such superheterodyne receivers, often termed midgets orsub-midgets, usually embody a rst detector circuit, intermediatefrequency amplifier, a second detector, an audio frequency amplier,reproducer, and a certain amount of radio frequency amplificationbetween the antenna and the rst detector circuit. However, because ofthe economy requirements in the construction of such low pricedreceivers, the degree of selectivity between the antenna and the rstdetector is not comparable to the relatively higher degree ofselectivity in a more expensive superheterodyne receiver. Y

One of the undesirable results of such insulicient radio frequency aheadof the first detector resides in annoying response due to heterodyn- 'Aing between undesired carriers. A particularly disagreeable type ofundesired response in a superheterodyne receiver, where economy ofconstruction is the prime consideration, occurs when two undesiredcarriers are impressed on the antenna circuit which differ byapproximately the intermediate frequency. employed. The insuflicientradio frequency selectivity ahead of the `first detector permits theseundesired signals to reach the first detector where ythey heterodyne,and thereby cause an intermediate frequency component. The lattercomponent in turn heterodynes with the desired intermediate frequencysignal which has been produced by the Aheterodyning of the localoscillation and the desired radio frequency carrier to which the radiofrequency system is tuned.

Now, I have discovered a novel method of, and devised means for,suppressing undesired responses in a low priced superheterodyne receiverwherein the undesired responses are caused particularly by undesiredcarriers differing by` approximately the operating intermediatefrequency, and which undesired carriersv cannot be blocked by sufficientradio frequency selectively ahead of the first detector.

Accordingly, it may be stated that it is one pf lthe prime objects ofthe present invention lto provide a superheterodyne receiver of the`type employing tuned radio frequency amplification ahead of. the firstdetector and intermediate frequency amplification between the rst andsecond detectors, wherein the ratio of intermediate frequency gain toradio frequency gainY is maximized to avoid undesired responses due to.heterodyning produced by two undesired carriers differing byapproximately the operating intermediate frequency. Y

Another important object of the present invention may be `said to residein the provision of a superheterodyne receiver, of the type whereineconomy of price and construction is the main consideration, thereceiver being characterized by the fact that its intermediate frequencyamplier gain'bears a predetermined relation to its radio frequencyamplifier gain, and this relation is such Vthat undesired responsecaused by heterodyning of two undesired carriers differing byapproximately the operating intermediate frequency is substantiallyeliminated.

Another object of the present invention is to provide in asuperheterodyne receiver, a device for simultaneously decreasing radiofrequency amplification prior to the first detector and increasingintermediate frequency amplification between the first and seconddetectors in a ratio such that there is substantially suppressedundesired responses due to the heterodyning of two undesired carriersdiffering by approximately the operating intermediate frequency.

Other objects of the present invention areto improve generally theefiiciency of low priced superheterodyne receivers, and to particularlyprovide a receiver of this type which is not only economicallyconstructed and marketed, but reliable in operation and free fromundesired responses.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings,

Fig. 1 shows generally a superheterodyne receiver embodying the presentinvention,

Y Fig. 2 shows a graphical explanation of the present invention,

4Fig. 3 shows aY modified form of the invention,

Fig. 4 shows still another modification,

Fig. 5 shows still another modified form of the invention.

Referring now to the accompanying drawings wherein like referencecharacters designate similar circuit elements in the different figures,there is shown in Fig. 1 a superheterodyne receiver, which is to beunderstood as being of the so-called midget type, the receiver embodyingthe usual grounded antenna circuit A, a radio frequency amplifier havinga tunable input, rst detector having a tunable input, a tunable localoscillator, an intermediate frequency amplifier, and a second detectorwhose output is adapted to be coupled to an audio frequency amplifierand a reproducer. The tuning devices, such as variable tuningcondensers, in the radio frequency amplier, rst detector and localoscillator circuits are arranged to be uni-controlled as -shown by thedotted lines 1, in any well known manner. The tunable circuits of theradio frequency amplier and first detector. stages vare tuned to acommon signal frequency, and this differs from the local oscillatorfrequency by the desired operating intermediate frequency. Theconstruction of a superheterodyne receiver of the type shown in Fig. 1is too well known to those skilled in the art to require any furtherdescription.

In low priced superheterodyne receivers it is impossible to providesufficient radio frequency selectivity ahead of the first detectorbecause of cost considerations. It has been found that a particularlyannoying type of undesired response occurs in such a low cost receiverwhen the antenna circuit picks up two undesired carriers which differ byapproximately the operating intermediate frequency. The insufficientradio frequency selectivity ahead of the first detector permits thesetwo undesired signals to reach the first detector. There they heterodyneand produce an intermediate frequency component which heterodynes withthe desired intermediate frequency signal which has been produced by theheterodyning of the local oscillation and the desired radio frequencycarrier to which the radio frequency amplifier and rst detector aretuned.

Now, it can be shown that the undesired intermediate frequency componentdue to the two undesired carriers, say A and B, is proportional to theproduct of the intensities of A and B. If

the gain of the radio frequency amplifier tube is halved, the strengthof this undesired intermediate frequency component caused by A and B isquartered. The intermediate frequency ampliiier gain is now doubled inorder to maintain the original desired signal sensitivity. It will nowbe seen that the result of halving the radio tical, until limited byother considerations.

frequency gain and doubling the intermediate frequency amplifier gain isto halve the strength of the undesired intermediate frequency componentat the second detector.

Similarly, when the radio frequency amplifier gain is decreasedfour-fold, and the intermediate frequency amplier is increasedfour-fold, the undesired intermediate frequency component, due to theproduct of A and B, is quartered. Therefore, as a fundamental principlein the design of a low cost superheterodyne receiver of the type shownin Fig. 1, there should be employed as low a radio frequency stage gain(grid of radio frequency tube to grid of first detector tube) and ashigh an intermediate frequency gain (over-all intermediate frequencysystem including conversion gain of first detector) as feasible, orprac- This has been found very desirable n the efficient 0peration, withminimum undesired response, of low cost superheterodyne receivers. Inother words, it will be seen that the essential design principleinvolves the maximization of the quantity intermediate frequency gain.radio frequency gain In Fig. 2 there is shown charted, in a purelyillustrative manner, the eect of the present invention in reducing theoperating disadvantage already described. Under the title OriginalCondition there is shown in Fig. 2 the quantitative relation between thedesired intermediate frequency component and the undesired intermediatefrequency component whose effect is to be minimized. It will be notedthat the desired intermediate frequency component is the product of thedesired signal and the local oscillation, while the undesiredintermediate frequency component is the product of the undesiredcarriers A and B. The ratio of desired intermediate frequency componenttc undesired intermediate frequency component is 8:1 in the examplegiven in Fig. 2. Of course, the intensity units, and

ance with the present invention. It will be notil;

ed that the desired signal has been reduced to half of its originalintensity, and the product, or desired intermediate frequency componentis now 16 units. However, each of the undesired carriers A and B havebeen reduced to one unit in in- "j tensity, and their product istherefore one unit. Hence, it will be seen that the ratio between thedesired and undesired intermediate frequency components is now 16:1.

Under the title AMPL. there is shown how the original signal sensitivityis restored by increasing the intermediate frequency amplification. Thedesired intermediate frequency component is now restored to its original32 units, while the undesired co.l f '1.

ponent has been only increased to 2 units. Therefore, it will be seenthat although the original signal sensitivity has been restored, theratio of desired to undesired intermediate frequency component is still16:1. In this way the present invention enables the undesired responsedue to the heterodyning of the undesired carriers A and B to besubstantially suppressed, while the original signal sensitivity issubstantially maintained. As has been pointed out heretofore thereceiver shown in Fig. 1 may have its radio frequency amplifier andintermediate frequency amplifier originally designed to accomplishtheeffects graphically shown in Fig. 2.

In Fig. 3 there is shown a modification of thev present inventionwherein a superheterodyne receiver, of the type shown in Fig. l, mayinclude, in addition to the uni-control tuning mechanism 1, and themanual volume control (M. V. C.)

Effect of Increased I. F.

2, a device for suppressing undesired response of radio frequencyamplifier tube 4, and a secondy ICH variable resistor device 5 connectedbetween the grid and cathode of the intermediate frequency amplifiertube 6. A uni-control means 7, shown in dotted lines, may be connectedin any desired manner to mechanically couple the sliders 8, 9 of thedevices 3 and 5 so that when the slider 8 moves in the direction of thearrow, to increase the negative bias of tube 4, the slider 9 moves inthe opposite direction to decrease the negative bias on the grid of tube6.

In this way the effects graphically charted in Fig. 2 may be secured. Inother words, the device 7 functions as a manual gain control for theradio frequency and intermediate frequency ampliers, which gain controlis independent of the manual volume control device 2 disposed betweenthe output ofthe second detector and the input of the audio frequencyamplifier. The number of sliders 8 and 9 may be so arranged that whenthe gain of the radio frequency amplifier tube 4 is reduced by half thenthe gain of the tube 6 would be doubled. Of course, the devicev 7 wouldnot be actuated by the operator of the receiver until the type ofinterference discussed above would make itself manifest. The operationof this form cf the invention is believed to be clear from thedescription of Fig. 2.

Fig. 4 shows a modification of the arrangement in Fig. 3 wherein asingle grounded slider l0 is employed to cooperate with a singleresistor l1 to vary the gain of the radio frequency and intermediatefrequency amplifier tubes 4 and 6. Only such portions of the circuit areshown in Fig. 4 which are necessary to an understanding of theinvention. rThus, the first detector cir cuit is shown in Fig. 4conventionally as a detector-oscillator stage, the construction of thelatter being weil known to those skilled in the art. In Fig. 3, thisstage is generally designated as a frequency changer.

The variation in Vgain of the tubes 4 and 6 in Fig. 4 is produced byconnecting the resistor ll between the cathodes of tubes 4 and 6, andbiasing the control grids of these tubes with respect to the cathodes.Thus, when the tap 16 is moved along the resistor l1 towards the cathodeof tube 6, then the negative bias of the grid of tube 4 with respect toits cathode is increased, while the negative bias of the grid of tube 6is decreased.

The manual volume control in this figure is shown of a different type,and involves the use o f a variable resistor l2 disposed in the screengrid circuit of tube 6. The operation of the arrangement shown in Fig. 4is similar to that described in connection with Figs. 2 and 3.

In Fig. 5 there is sho-wn still another possible modification of thearrangement shown in Figs. 3 and 4. Here, the amplitude of the radiofrequency energy impressed on the detector-oscillator stage iscontrolled by varying the radio freo the grid of the intermediatefrequency amplifier tube 6.

Here, again, when the slider l0 is moved along the resistor 11 towardsthe cathode of tube 6, the radio frequency signal input to the radiofrequeni `cy amplier is reduced, while the intermediate frequencyamplifier gain is increased. Hence, it will be seen that the operationof this modification of the invention results in the effects describedin connection with Fig. 2. By reducing the amount of radio frequencysignal energy collected by the antenna circuit to one half, for example,and simultaneously doubling the intermediate frequency amplifier gain,the ratio of desired intermediate frequency component to undesiredintermediate frequency component will be doubled. Of course, as statedin connection with Fig. 3, the slider 10 would not be adjusted until thetype of undesired response referred to arises and is reproduced by thereproducen In the modification shown in Figs. 3, 4 and 5 it is to beclearly understood that adjustment of the undesired response suppressormay be made to cover a wide range of conditions. That is, the suppressordevice could be adjusted to conform to the intensity of the undesiredresponse. It is believed that the range of movement thereof will beobvious from the aforegoing description, and it is to be clearlyunderstood that this range of adjustment may be as wide as desired, oras small as desired, depending upon the undesired responses sought to besuppressed.

Moreover, while I have indicated and described several systems forcarrying my invention into effect, it will be apparent to one skilled inthe art that my invention is by no means limited to the particularorganizations shown and described, but that many modifications may bemade without departing from the scope of my invention as set forth inthe appended claims.

What I claim is:

1. A method of reducing undesired response in superheterodyne reception,which undesired response is produced by the heterodyning of at least two`undesired carriers differing by substantially the operatingintermediate frequency, which consists in collecting the undesiredcarrier energy and the desired signal energy, reducing the amplicationof the said collected energies substantially below a predeterminednormal amplification level, impressing the amplified energies upon a 1.by substantially the operating intermediate frequency, which consistsin simultaneously collecting desired signal energy and said undesiredcarrier energies, amplifying the said collected energies, impressing theamplified collected energies upon a frequency changer to produce adesired intermediate frequency component which is derived from saiddesired signal frequency energy and an undesired intermediate frequencycomponent which is the product of the said two undesired carrierenergies, amplifying the desired and undesired intermediate frequencycom-ponents, and simultaneously reducing the said first amplificationand increasing the said second ampliiication in a predetermined mannerto substantially increase the ratio of said desired component to saidundesired component when the intensity of the latter becomesundesirable.

3. In combination, in a superheterodyne receiver,a firstdetectona seconddetector,aninter mediate frequency amplifier connected between 'thefirst and second detectors, and a radio frequency amplifier connected tothe input of the first detector, the selectivity of the amplifierpreceding said first detector being insufficient to prevent theproduction of undesired intermediate 'frequency component caused by atleast two undesired carriers differing by substantially the operatingintermediate frequency, and the gain of said radio frequency amplifierbeing of such a magnitude with respect to the gain of the intermediatefrequency amplifier that the ratio of desired intermediate frequencycomponent to the said undesired component is greatly increased.

4. In combination, in a superheterodyne receiver, a first detector, asecond detector, an intermediate frequency amplifier connected betweenthe first and second detectors, and a radio frequency amplifierconnected to the input of the first detector, the selectivity of theamplifier preceding said first detector being insufficient to preventthe production of undesired intermediate frequency component caused byat least two undesired carriers differing by substantially the operatingintermediate frequency, and means for adjusting the gain of said radiofrequency amplifier to such a magnitude with respect to the gain of theintermediate frequency amplifier that the ratio of the desiredintermediate frequency component to the said undesired component isgreatly increased.

5. In combination, in a superheterodyne receiver, a rst detector, asecond detector, an intermediate frequency amplifier connected betweentlie first and second detectors, and a radio frequency amplifierconnected to the input of the first detector, the selectivity of theamplifier preceding said first detector being insufficient to preventthe production of an undesired intermediate frequency component causedby at least two undesired carriers differing by substantially theoperating intermediate frequency, and means for decreasing the gain ofsaid radio frequency amplifier to such a magnitude and increasing thegain of the intermediate frequency amplifier to such an extent that theratio of the desired intermediate frequency component to the saidundesired component is greatly increased.

6. In combination, in a superheterodyne receiver, a iirst detector, asecond detector, an intermediate frequency amplifier connected betweenthe nrst and second detectors, and a radio frequency amplifier connectedto the input of the first detector, the selectivity of the amplierpreceding said first detector being insuiiicient to prevent theproduction of an undesired intermediate frequency component caused by atleast two undesired carriers differing by substantially the operatingintermediate frequency, and a single means for decreasing the gain ofsaid radio frequency amplifier to such a magnitude and increasing thegain of the intermediate frequency amplifier to such an extent that theratio of the desired intermediate frequency component to the saidundesired component is greatly increased.

7. In a superheterodyne receiver, a frequency changer, means forcollecting signal energy of the broadcast frequency range, a selectivecircuit, including a frequency selection means, coupling said collectingmeans and said frequency changer, an intermediate frequency amplifiercoupled to said frequency changer, the selectivity of said Vselectivecircuit being insufficient to prevent undesired carriers from reachingthe frequency changer and producing a heterodyne frequency which is of afrequency substantially equal to the operating intermediate frequency,and means for simultaneously decreasing the intensity of the desiredsignal and undesired carrier energies impressed on said frequencychanger to a predetermined fraction, and increasing the gain of saidintermediate frequency amplifier by a number of times equal to thedenominator of said fraction when the intensity of said undesiredintermediate frequency component becomes undesirable.

8. In combination, in a superheterodyne receiver, a frequency changercircuit, a second detector circuit, an intermediate frequency amplifiercoupling said frequency changer and second detector, a radio frequencytransmission network coupled to the input of said frequency changer, anda single means for simultaneously decreasing the transmission efciencyof said radio frequency network and increasing the gain of saidintermediate frequency amplifier in a predetermined manner with respectto said transmission efficiency reduction.

9. In a superheterodyne receiver, a first detector circuit, a seconddetector circuit, an intermediate frequency amplifier coupling the firstand second detectors, a radio frequency amplifier connected to the inputof said first detector, and means for simultaneously decreasing the gainof said radio frequency amplifier and increasing the gain of saidintermediate frequency amplifier.

WILLIAM S. BARDEN.

